1. Field of the Invention
The present invention relates to a silicon carbide semiconductor device and a manufacturing method of the silicon carbide semiconductor device.
2. Description of the Related Art
Among semiconductor devices conventionally used as power devices, those using silicon (Si) as a semiconductor material have become mainstream. Compared to silicon, silicon carbide (SiC) has a wider bandgap (hereinafter, wide gap semiconductor) and physical properties such as a thermal conductivity that is 3 times that of silicon, a critical electric field strength that is 10 times that of silicon, and electron drift velocity that is 2 times that of silicon. Therefore, research concerning the application of silicon carbide in the production (manufacturing) of a power device that is capable of high temperature operation with low loss and for which the dielectric breakdown voltage is high, is being actively pursued by institutions.
A mainstream structure of such a power device is a vertical semiconductor device having a back surface electrode equipped with a low resistance ohmic electrode in the back surface side. Various materials and structures are used for the back surface electrode of a vertical semiconductor device. For instance, surface electrodes have been proposed that are respectively formed by stacked layers including a titanium (Ti) layer, a nickel (Ni) layer, and a silver (Ag) layer (for example, refer to Japanese Laid-Open Patent Publication No. 2007-184571); and a titanium layer, a nickel layer, and a gold layer (for example, refer to Japanese Laid-Open Patent Publication No. 2010-86999).
For example, a method has been proposed where, in a vertical semiconductor device that uses silicon carbide and is typified by a Schottky barrier diode (SBD), a nickel layer is formed on a semiconductor substrate of silicon carbide (hereinafter, SiC substrate) and thereafter, the nickel layer is subjected to heat treatment to form a nickel silicide layer, whereby contact (electrical contact unit) of the SiC substrate and the nickel silicide layer is an ohmic contact (for example, refer to Japanese Laid-Open Patent Publication Nos. 2007-184571 and 2010-86999). Nonetheless, in Japanese Laid-Open Patent Publication Nos. 2007-184571 and 2010-86999, a problem arises in that when the back surface electrode is formed on the nickel silicide layer, the back surface electrode is prone to peel from the nickel silicide layer.
To resolve such a problem, a method has been proposed where after the nickel layer remaining on the surface of the nickel silicide layer has been removed exposing the nickel silicide layer, the back surface electrode is formed by sequentially stacking on the nickel silicide layer, the titanium layer, the nickel layer, and gold layer, whereby the peeling of the back surface electrode is suppressed (for example, refer to Japanese Laid-Open Patent Publication No. 2008-53291). Another method has been proposed where after a metal carbide formed on the surface of the nickel silicide layer has been removed, the back surface electrode is formed on the nickel silicide layer, whereby the adhesion of the back surface electrode is improved (for example, refer to Japanese Laid-Open Patent Publication No. 2003-243323).
Nonetheless, even if the back surface electrode is formed using the techniques of Japanese Laid-Open Patent Publication No. 2008-53291 or Japanese Laid-Open Patent Publication No. 2003-243323, the adhesion of the nickel silicide layer and the titanium layer, which is the lowest layer of the back surface electrode, is poor and when the semiconductor wafer is diced (cut) into chips, the back surface electrode peels from the nickel silicide layer. For example, in Japanese Laid-Open Patent Publication No. 2008-53291, the nickel layer is formed on the SiC substrate and thereafter, heat treatment is subsequently performed, whereby the nickel silicide layer is formed, and the ohmic contact of the SiC substrate and the nickel silicide layer is formed. According to Japanese Laid-Open Patent Publication No. 2007-184571, the nickel silicide layer is generated by a solid-phase reaction of silicon carbide and nickel, indicated below as equation (1).Ni+2SiC→NiSi2+2C  (1)
Carbon (C) resulting from the reaction of equation (1) exists dispersed throughout the nickel silicide layer as very fine precipitate or in an over-saturated state in which the crystals are unstable. The carbon dispersed in the nickel silicide layer is discharged all at once and precipitates (agglomerates) in a layer as a precipitate, such as graphite, on the surface of and inside the nickel silicide layer by heat treatment performed after the formation of the nickel silicide layer. The precipitate resulting from the agglomeration of this carbon is brittle and has poor adhesion; and therefore, even with the slightest stress, easily fractures, whereby the back surface electrode formed on the nickel silicide layer peels.